"You can be any shape you like," Erik points out with his usual directness, turning up a hand for emphasis. It's an interesting capacity of Raven's mutation; if she takes on the appearance of, say, a circus strongman, what does that mean for her actual muscle strength? Are her cells able to mimic that as well?
He didn't indulge further banter about Charles, although the corners of his mouth turned up with a fondness nearly uncharacteristically soft, or at least far away. In fact that was the precise reason that he didn't add a rejoinder; it was treading in potentially bloody waters even with Raven, as far as Erik was concerned. Much easier, and better, to focus on the present. "I'm offering to teach you self-defense. And offense, as necessary."
It was a rephrasing rather than restating, with a distinction he considered important. "But it's conditional." Of course it was, the wry turn of his grin allowing she wouldn't be surprised by as much. "I'll only teach the real Raven. It's a waste of time for both of us if you spend half your effort maintaining someone else's appearance."